1. Field of the Invention
The present invention relates generally to a zinc-halogen secondary battery for accumulating electric power. More specifically, the invention relates to a zinc-halogen secondary battery which is provided with an enhanced charge efficiency by avoiding the attachment of bubbles to electrodes during charging.
2. Description of the Prior Art
In the recent years, there have been developed a number of cell-type electric-power accumulation systems. Various zinc-halogen batteries (e.g., zinc-bromine batteries, zinc-chloride batteries and so forth) have been developed for use in such power accumulation systems. In cases where such batteries are used for accumulating electric power, a large electric power storage capacity is available, generated by circulating electrolyte and an external active material. However, in some technologies, such as uninterruptible power sources, a large electric power accumulation capacity is not required. In such cases, a non-flow type or fluid-static type battery is useful.
In such a battery, one problem encountered is that bubbles are generated and attach to the cathode electrode surface while the battery is in operation. Bubbles are formed by air heated with the electrolyte or by direct reaction of bromine (redissolved from the anode electrode) with zinc (extracted from the cathode electrode) which generates hydrogen to form hydrogen bubbles. In the alternative, when the amount of zinc in the vicinity of the cathode electrode upon electrodeposition of zinc becomes less than or equal to 0.5 mol/l, the generation of hydrogen is rapidly increased, forming bubbles.
When charging is conducted while bubbles are present, zinc cannot be deposited on the cathode electrode at portions where bubbles are attached, thus the smoothness of the cathode electrode surface is degraded and dendritic crystals may also form which cause degradation of the battery efficiency.
The bubbles can be removed by circulating the electrolyte by means of a fluid pump. In such cases, however, a shuttle current will flow through the manifold to cause a degradation in the uniformity of electrodeposition of zinc at a plurality of the cells and heat may be generated. Therefore, the bubbles can be removed by generating a flow of the electrolyte only when the battery voltage is zero.